Lyme borreliosis (Lyme disease and related disorders) is a zoonosis characterized by a number of variable syndromes. The etiological agent of this disease is the spirochete Borrelia burgdorferi, which is primarily transmitted by Ixodes ticks. The northern deer tick, Ixodes dammini is the major vector of Lyme disease in Minnesota, Wisconsin, the northeastern United States and adjacent Canada. The California black-legged tick, I. pacificus is the primary vector of this disease in the western United States, and in Europe the major vector of Lyme borreliosis is I. ricinus. The spirochete has also been found in deerflies, horseflies and mosquitos. The preferred hosts for the larval and nymphal stages of these ticks are small rodents such as Peromyscus leucopus, the white-footed mouse, whereas the adult ticks preferentially feed on large mammals, such as deer. Since transovarial transmission of the spirochetes by these ticks occurs infrequently, the disease is transmitted by the nymphal and adult ticks. The frequent isolation of B. burgdorferi from white-footed mice captured in foci of Lyme disease suggests that small rodents may serve as a natural reservoir for this spirochete and source of infection for the larval and/or nymphal stages of the tick. Local spread of the Ixodes ticks is by mammalian hosts, and birds may serve an important role in long distance tick dispersal.
Most cases of Lyme disease occur in June or July, when the aggressive nymphal stage is most active. As many as two-thirds of the people that become infected by this spirochete are unaware of the tick bite because of the painless bite and the small size (several mm) of the nymphal stage.
Spirochetes are introduced into the host at the site of the tick bite and this is also the location of the initial characteristic skin lesion, erythema chronicum migrans (ECM). A systemic illness ensues due to the lymphatic and hematogenous spread of B. burgdorferi. The early phase of the illness often consists of the ECM, headache, fatigue, muscle and joint aches, stiff neck and chills and fever. This phase of the disease may be followed by neurologic, joint or cardiac abnormalities. The chronic forms of the disease such as arthritis (joint involvement), acrodermatitis chronica atrophicans (skin involvement), and Bannwart's syndrome (neurological involvement) may last for months to years and are associated with the persistence of the spirochete. A case of maternal-fetal transmission of B. burgdorferi resulting in neonatal death has been reported. Domestic animals such as the dog also develop arthritis and lameness to this tick-borne infection. For every symptomatic infection, there is at least one asymptomatic infection. Lyme disease is presently the most commonly reported tick-borne disease in the United States.
The infection may be treated at any time with antibiotics such as penicillin, erythromycin, tetracycline, and ceftriaxone. Once infection has occurred, however, the drugs may not purge the host of the spirochete but may only act to control the chronic forms of the disease. Complications such as arthritis and fatigue may continue for several years after diagnosis and treatment.
Since the effectiveness of the present methods of treatment are limited, and vector control is impractical at best, a need exists for a vaccine which is effective to immunize high-risk individuals and susceptible domestic animals against Lyme borreliosis.